198 THE HEART. 



bridged by fine fibers. This space ultimately disappears and the inner and 

 outer tubes unite to form the definitive walls of the heart. 



The volume and complexity of this primordial heart tube, its early appear- 

 ance, and its wide separation from the myocardium are most remarkable. These 

 conditions are not satisfactorily explained by the assumption that the cardiac 

 endothelium is a secondarily acquired investment of a primitive muscular heart 

 tube. Limulus has the largest and most complex heart of any living arthropod, 

 and if an endothelium layer is present in any invertebrate, it should be present 

 there, but a careful search in both adult and embryonic hearts failed to reveal any 

 trace of such a layer. It is possible that in vertebrates, the cardiac and peri- 

 cardiac walls of the arachnids have united to form a two-layered heart. But this 

 would not account for the presence of the cardiac ganglia on the outer surface of 

 the myocardium in vertebrates. 



An alternative and preferable hypothesis would be to assume that the arach- 

 nid heart represents the ventricular portion in vertebrates, and that the posterior 

 posterior portion of the pericardia! chamber represents the thinwalled auricu- 

 lar or atrial portion and the venous sinus. (Fig. 44.) 



III. CIRCULATION. 



Arachnids. The circulation in Limulus has reached a very high stage of devel- 

 opment. Milne Edwards, in his classic work on the anatomy of Limulus, says, 

 "The circulatory system of Limulus is more perfect and more complicated than 

 in any other arthropod. The venous blood, instead of being distributed in inter- 

 organic lacunae, as in the Crustacea, is, through a great part of its course, en- 

 closed in special vessels, having walls distinct from the adjacent organs, and 

 often rising by branches of remarkable delicacy and passing into chambers, well 

 circumscribed for the most part. The nourishing fluid passes from these reservoirs 

 into the gills, and hence, by a system of branchio-cardiac canals, to the peri- 

 cardial chamber and the heart, which is very large. It is then forced into the 

 tubular, resisting arteries, which have a most complex arrangement, with 

 frequent anastomoses and with terminal ramifications of marvellous tenuity and 

 richness, and which can be followed even in the most delicate membranes." 



The heart of Limulus is a voluminous muscular tube, ending blindly behind. 

 It is provided with eight pairs of slit-like openings, or ostia, each opening guarded 

 by two semilunar valves through which the blood enters the heart from the peri- 

 cardial chamber. (Figs. 115-118.) The blood is pumped forward, and escapes 

 through three pairs of aortae, one pair of cerebral arteries, and a frontal artery. 

 The three terminal arteries are guarded by one very large valve on the haemal 

 wall of the heart. The walls of the heart consist of a loosely felted mass of inter- 

 woven muscle bands, without any recognizable endothelium. (Fig. 2.) 



Vertebrates. Comparison of the circulation in Limulus with that in verte- 

 brates is difficult. There are some striking resemblances and some equally 

 striking differences. 



